In a known yarn braking device (EP 0 534 263 A) a mechanical spring constitutes both the axial force generator and the radial centering device. The spring may be an annular radially oriented diaphragm, a radial spiral spring, a conical spiral spring, a cylindrical bellows, or, as shown in FIG. 1 of EP 0 652 312 A, a star-shaped spring arrangement consisting of helical tension springs each of which is hooked into the holder and into the support ring body, respectively. A general problem of mechanical springs is a development of the force which is not uniform in circumferential direction, the susceptibility to aggressive substances, and a tendency to collect lint. A further problem is that the mechanical spring at the same time has to centre in radial direction and has to transmit the axial force on the braking body. This dual function means a compromise between the development of the resilient axial force and of the radial centering force and might be critical in cases of extreme braking effects, i.e. if the same reliable centering of the frustocone braking body is necessary in case of an extremely weak braking effect or in case of an extremely strong braking effect. The adjustment range of the braking effect is limited by the nature of the mechanical spring, meaning that the mechanical spring has to be substituted by another as soon as a significant variation of the braking effect is needed. Basically, the braking effect is adjusted by the axial position of the holder in relation to the withdrawal end in order to load the spring more or less. In the case of a very weak braking effect due to the low spring load the centering and automatic return of the dislocated braking body into the centered position may fail, while in the case of an extremely strong adjustment of the braking effect the centering may be too rigid due to the high spring load. An optimal and constant centering effect and the capability of the braking body to automatically return after occurrence of a needed lateral displacement into a perfectly centered position on the withdrawal end of the braking body is, however, a decisive prerequisite for a correct braking function, since the large diameter end region of the frustocone braking body only then is able to produce a uniform braking effect along the circumference of the withdrawal end when the small diameter end of the frustocone braking body remains perfectly centered. Already small misalignments results in permanent fluctuations of the braking effect and in undesirable variations of the yarn tension. The yarn which rotates during withdrawal from the storage body in the yarn braking device like the hand of a clock in most cases is deflected in the support ring body and then applies a rotating, outwardly directed force on the braking body which force is varying, e.g. in case of a passing knot, and which has to be taken up and compensated permanently by the centering device. For that reason a properly operating centering device has a significant functional importance for this kind of a yarn braking device.
It is known from DE 195 31 579 A in a small diameter circular disc brake, which the yarn is only passing laterally, to press the braking discs against each other by axially repelling permanent magnet rings. However, due to the only linearly passing yarn the functional requirements for centering are low since the discs are centered mechanically and are inclined in relation to each other during operation.
Furthermore, it is known for controlled yarn braking devices (DE 198 39 272 A, EP 0 652 312 A, U.S. Pat. No. 5,778,943 A), the braking effect of which either can be modulated or can be switched off completely, to provide a magnetic axial force generator for a basic braking effect or passive position in combination with a mechanical spring arrangement. The axial force generator comprises at least one coil which is supplied with current. In the deenergised condition the axial force generator does not generate any force.
It is an object of the invention to provide a non-controlled yarn braking device of the kind mentioned in the beginning which is structurally simple and reliable, allows a broad adjustment range of the braking effect and which has good performance even in case of extremely weakly and extremely strongly adjusted braking effects.
This object is achieved according to one embodiment by providing a yarn braking device for a yarn feeding device, the yarn braking device having an axially stiff, radially deformable braking body with the shape of a frustocone, the large diameter end of the braking body being set coaxially over a rounded withdrawal end of a drum-shaped storage body and being pressed resiliently against the withdrawal end from the small diameter end by an axial force defining the braking effect between the braking body and the withdrawal end. An axial force generator acting in the axial direction and a centering device acting in the radial direction are also provided, respectively, between a stationary holder and the braking body. The axial force generator is formed by at least one pair of permanent magnets, the permanent magnets of which are aligned axially to each other by the centering device with an intermediate gap, and the centering device includes an axial sliding guiding system which is separated structurally and functionally from the pair of permanent magnets.
Pursuant to an additional embodiment of the invention, a yarn braking device for a yarn feeding device is provided, the yarn braking device having an axially stiff, radially deformable braking body with the shape of a frustocone, the large diameter end of the braking body being set coaxially over a rounded withdrawal end of a drum-shaped storage body and being pressed resiliently against the withdrawal end from the small diameter end by an axial force defining the braking effect between the braking body and the withdrawal end. An axial force generator acting in the axial direction and a centering device acting in a radial direction are also provided, respectively, between a stationary holder and the braking body. The axial force generator and the centering device at the same time are formed by at least one pair of permanent magnets, of which one inner permanent magnet is supported against the holder while the other outer permanent magnet of the pair is supported against the braking body, and the permanent magnets of the pair are aligned to each other via an intermediate gap and such that the direction of the action of the magnet force is inclined obliquely towards the axis of the yarn braking device. Further, the permanent magnets of the pair are generating both axial force components and also radial force components, respectively.
In accordance with the first embodiment, the pair of the permanent magnets operates without contact and with a function which is not liable to aging, to aggressive substances, to misalignments, does not tend to develop the force irregularly, and which assures a wide adjustment range for the braking effect. The pair of permanent magnets exclusively has to generate the resilient axial force which determines the braking effect while the needed centering of the frustocone braking body is carried out at the small diameter end section by the sliding guiding system. The produced centering effect is the same for all adjustments of the braking effect. Both functions, i.e. the generation of the axial resilient force and the axial guidance may by optimised respectively per se since these functions do not interfere with each other during the operation of the yarn braking device. The problem of lint collection and the negative influence of collected lint are eliminated. The structural construction of the yarn braking device is simple and results in high reliability as there are no liable mechanical spring components.
In the solution according to the second embodiment, the pair of permanent magnets at the same time forms the axial force generator and the centering device, i.e., the small diameter end of the braking body is supported without contact by magnet forces only, and at the same time is axially actuated against the storage body and is radially actuated from all sides in the direction towards the axis of the yarn braking device by radial force components of the magnet effect, and is centered accordingly. Since there is no mechanical contact the yarn braking device is characterised by a prompt and precise response behaviour. The at least one pair of permanent magnets in the yarn braking device forms, so to speak, a virtual or magnetic spring. The respective inner permanent magnet could be provided directly in the braking body or could be integrated even into the material of the braking body, respectively.
As it is decisive for the desired braking function that the precisely adjustable axial resilient force permanently actuates the always correctly centered frustocone braking body against the withdrawal end, the permanent magnets in the pair of permanent magnets could be provided such that they either repel or attract each other, and such that the available mounting space is optimally used.
In case of single pairs of permanent magnets at least three regularly distributed pairs should be provided.
Very uniform development of the force can be achieved by ring-shaped permanent magnets which co-act essentially on the same diameters or even on different diameters.
Alternatively, e.g. for weight reasons, more than three permanent magnet pairs each consisting of single permanent magnets could be distributed in circumferential direction. In this case either a provided axial sliding guiding system will form an anti-rotation mechanism for the permanent magnets within the pairs in order to always align the permanent magnets to each other, or the single permanent magnets could be designed such or/and arranged such that they automatically generate an anti-rotation effect by the magnetic co-action.
The support ring body of a specific embodiment in which the centering device simultaneously constitutes the anti-rotation mechanism, is held in an outer ring carrying at least three axial guiding pins which are distributed in circumferential direction. The support ring body carries either a ring-shaped permanent magnet or several single permanent magnets, respectively. The holder is formed with a ring section which is equipped with guiding sleeves for the guiding pins and which either is provided with a ring-shaped permanent magnet or with single permanent magnets in a multiple arrangement. Alternatively, the guiding pins also may be anchored in the ring section of the holder, while the guiding sleeves then will be provided in the outer ring. The guiding pins should penetrate the guiding sleeves with a weak slide fit.
In a further expedient embodiment the outer ring is formed at the inner side with a conical seat for the small diameter end of the braking body. The support ring body is a snap ring which is snapped into the outer ring in order to position the braking body in the seat. This is advantageous in terms of assembly and allows, if needed, a prompt and comfortable replacement of the braking body.
In a further expedient embodiment the support ring body is secured at a small diameter ring edge of a generally conical cage the large diameter end region of which either is equipped with a ring-shaped permanent magnet or with several single permanent magnets, respectively, and which surrounds the braking body with radial distance. The cage is loosely inserted into a support ring which either includes the other ring-shaped permanent magnet or several single permanent magnets, respectively, and which is provided with axial holder feet which are distributed in circumferential direction. The inner sides of the holder feet define axial sliding guiding surfaces for a counter guiding surface at the outer periphery of the large diameter end region. In the case of ring-shaped permanent magnets an anti-rotation mechanism is not needed. To the contrary, an anti-rotation mechanism may be expedient in case of single permanent magnet pairs, e.g. between the cage and the support ring or between the sliding guiding surfaces and the counter guiding surface. The counter guiding surface may be concavely rounded in an axial section of the cage such that an axially shiftable universal joint or ball joint is formed between the counter guiding surface and the axial guiding surfaces of the holder feet. The universal joint or ball joint, respectively, allows the operation movements of the radially deformable braking body without interference and properly centers the small diameter end of the braking body.
With a view to a comfortable assembly the holder feet are snap holders having an integrated predetermined bending elasticity for a snap fixation at the ring section of the holder. The cage and the holder feet offer sufficient intermediate spaces such that lint does not collect there, or such that access is provided at any time for cleaning purposes or for an inspection.
With a view to easy assembly the support ring body should be formed with an outside seat for the small diameter end section of the braking body. The seat is bounded on one side by a shoulder such that the support ring body can be snapped into the ring edge of the cage in order to position the braking body. The seat could be formed partially or in its entirety in the ring edge of the cage.
In a particularly expedient embodiment which operates without a mechanical axial sliding guiding system each outer single or the ring-shaped permanent magnet is arranged in relation to the axis on a larger diameter than each inner single permanent magnet or the inner ring-shaped permanent magnet. The permanent magnets of the pair or of the pairs, e.g. respectively repelling permanent magnets, co-operate such that forces are generated which are directed obliquely to the axis and such that radial force components of the forces can be used for the centering while the axial force components are used to generate the resilient axial force. The trick of arranging the outer permanent magnet or the outer permanent magnets, respectively, on a larger diameter than the inner permanent magnet or the inner permanent magnets, respectively, results in the effect that the inner permanent magnet in case of a displacement outwardly away from the axis will be exposed to an increasing counter oriented radial force component and then is pressed back with force again in the direction towards the axis. That means that the respective maximum centering radial force component only is generated then when the inner permanent magnet tends to displace outwardly. In this fashions the inner permanent magnet or the inner permanent magnets, respectively, are captured in the magnetic fields of the outer permanent magnets or the outer permanent magnet, respectively, provided that the braking body is contacting the withdrawal rim of the storage body under axial force. The small diameter end of the braking body remains properly centered even in case of forces which act radially outwardly and originate e.g. from the deflection of the yarn at the support ring body or from the passage of a knot.
In a preferred embodiment the repelling surfaces of the repelling permanent magnets of the pair which repelling surfaces face each other, are inclined obliquely with respect to the axis, even, preferably, are formed conically, and are at least substantially parallel to each other. The radial and the axial force components are generated already by this design of the permanent magnets.
In an expedient embodiment having two ring-shaped permanent magnets the permanent magnets may be conical rings having a rectangular or trapezoidal cross-section. Already by this form of the permanent magnets the direction of the magnetic action is inclined obliquely towards the axis of the yarn braking device and uniformly along the circumference such that the multiple effect of radial force components and of axial force components is achieved. The radial force components act counter to an outward displacement of the small diameter end and increase the stronger the more the small diameter end is displaced outwardly.
In an expedient embodiment having single permanent magnets in several pairs distributed along the circumference the outer single permanent magnets are offset in circumferential direction relative to the inner single permanent magnets such that each outer single permanent magnet is directed into the gap between adjacent inner single permanent magnets or vice versa. Since then each inner single permanent magnet at the same time is actuated by the magnetic forces of two outer single permanent magnets from different directions the co-acting permanent magnets automatically constitute a contact free magnetic anti-rotation protection mechanism. Also in this case the inner single permanent magnets ought to be arranged on a smaller diameter than the outer single permanent magnets in order to achieve the necessary centering and return functions.
In an expedient embodiment the support ring body carries the single inner permanent magnets or the ring-shaped inner permanent magnet, respectively. A conical support cage which grips over the small diameter end of the braking body and which is secured, preferably detachably, at the holder carries the single permanent magnets or the ring-shaped outer permanent magnet, respectively, on a carrying ring. This solution is of advantage with a view to easy manufacturing and easy assembly.
In a further expedient embodiment a cylindrical extension of the frustocone is formed at the small diameter end of the braking body. This measure avoids local overloads at the small diameter end when actuated by the axial force and allows a simple assembly e.g. by only tucking the braking body loosely into the support ring body.
In a further expedient embodiment an essentially cylindrical extension is provided at the support ring body. The cylindrical extension extends through the carrying ring of the support cage without contacting the carrying ring. This measure stiffens the support ring body and allows to limit the displacement of the braking body in an emergency case under extreme sideward displacement. During normal operation of the yarn braking device, however, there will not be any contact between the extension and the carrying ring.
It is important for the above-mentioned reasons that an intermediate distance is generated in the direction of the magnet action between the support ring body and the carrying ring of the support cage, the intermediate distance being at least as large as the size of the air gap between the permanent magnets.
An advantageous handling is achieved when the cylindrical extension of the support ring body at the end protruding beyond the carrying ring of the support cage is equipped with an outwardly directed catching projection, e.g. a ring flange the outer diameter of which is slightly larger than the inner diameter of the carrying ring. During assembly the support ring body first is put against resistance into the carrying ring. During the normal operation of the yarn braking device, i.e., as soon as the braking body abuts at the storage body, the catching projection does not engage at the carrying ring. However, during assembly or during transport, the engagement of the catching projection at the carrying ring assures that the support ring body and the braking body cannot fall out of the carrying ring.
The magnitude of the axial force of the axial force generator is adjusted by the axial position of the holder in relation to the withdrawal end of the storage body. In order to allow to change the adjusted magnitude of the axial force generated between the permanent magnets precisely and remotely controlled and without manual engagement at the adjustment device of the holder, in an expedient embodiment at least one coil is functionally associated to one of the permanent magnets of the axial force generator in order to allow to generate an auxiliary magnet force which is superimposed on the axial force by selectively supplying current to the coil. The auxiliary magnet force increases or reduces the axial force to a desired extent. So to speak, one of the permanent magnets provided anyway for the suspension of the braking body is used as an armature of a selectively controlled electromagnet. Since the permanent magnets generate a relatively strong axial force a coil and/or a moderate current may be sufficient, which are not particularly strong, to adjust in some cases only a weak increase or decrease of the axial force. The axial effect of the coil or of several coils can be amplified by correspondingly placed iron, preferably soft iron. This embodiment is particularly expedient for a knitting machine, in particular a circular knitting machine at which frequently many yarn feeding devices are installed and where during operation fluctuations in the quality of the knitted fabric may occur which promptly could be compensated for by a change of the braking effect or the knitting yarn tension, respectively. By means of the coils in the yarn braking devices then the axial forces can be changed independently from the value of the respective axial force in one group of or in all yarn feeding devices, respectively, such that by this measure and substantially at the same time the tensions in the knitting yarns are raised or lowered by essentially the same amount.
In a further embodiment the coil is arranged stationarily outside of the braking body and in association to the permanent magnet of the axial force generator which permanent magnet is supported at the braking body. In this case the permanent magnet provided anyway in the axial force generator is used without additional measures for this additional function.
In a further embodiment the coil is supported at the braking body and is functionally associated to the permanent magnet which is provided outside the braking body. The coil is lightweight such that the mass of the braking body remains low. The permanent magnet provided outside the braking body anyway is part of the axial force generator and can be used for this additional function without additional structural measures.
In a yarn braking device the braking body of which is arranged via a support ring body in a support cage the coil expediently is provided in the support cage or at the support ring body, respectively. Thanks to this placement the coil is located optimally close to the permanent magnet.